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The Met Police and Police Scotland have been working on an investigation linking the packages.

A joint statement said: "Given the packages received last week bore similarities to devices sent in the past which were linked to dissident groups associated with Northern Ireland-related terrorism, officers were already looking at this as a line of enquiry.

"However, we continue to keep an open mind and enquiries continue.

"We are also aware that those claiming responsibility have indicated five devices were sent. At this time, only four devices have been recovered.

"Extensive advice has already been issued to relevant businesses and sectors to be vigilant for and report suspicious packages to police.

"This advice was previously sent to armed forces personnel and is being reiterated again in light of this claim.

"We continue to urge the public to remain vigilant and report anything suspicious to police."

Police cordoned off an area at Waterloo station near to where a suspicious package was found (Image: REUTERS)

The one sent to Waterloo (Image: LONDON METROPOLITAN POLICE/HANDOUT/EPA-EFE/REX/Shutterstock)

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Workers were evacuated but the package was left untouched until police arrived and made the device safe.

No transport was disrupted despite the evacuations.

Detectives from the Met's Counter Terrorism Command are leading the investigation into the three packages received in London, whilst Police Scotland, under direction from the Crown, is leading the investigation into the package received at the University of Glasgow.

Both investigation teams are working closely together to share any information or intelligence that could assist their respective inquiries.